Heretofore, biomedical materials especially useful commercially as contact lenses have been based upon polymers and/or copolymers of a select set of chemistries. Methylmethacrylic acid type chemistries form the oldest type, the polymers from which poly(methylmethacrylates) (PMMA) have been surpassed by hydrogel chemistries based upon poly(hydroxyethyl methacrylate) (pHEMA) or polyvinyl pyrrolidinone (pNVP), and copolymers of HEMA and NVP. These materials formed the basis for most soft contact lenses.
Silicone chemistries have played a lesser role in the marketplace, but have offered higher oxygen permeabilities than their hydrogel counterparts. They have, however, presented certain performance characteristics which limit their application--specifically surface wettability problems.
Copolymers employing PMMA types of chemistry have been employed in conjunction with silicone chemistry and hydrogel chemistry to produce a wide assortment of materials which have the common characteristics of high oxygen permeability and high modulus (rigidity). These materials have been characterized as hard gas permeable or rigid gas permeable materials.
Urethane chemistries have not been employed commercially in the contact lens market despite significant work in the area such as U.S. Pat. No. 3,786,034 issued to Blair et al relates to hard, hydrophilic polyurethane material formed from reacting a specific polyol with a polyfunctional isocyanate. U.S. Pat. No. 3,821,186 teaches similar materials as U.S. Pat. No. 3,786,034.
U.S. Pat. No. 4,136,250 teaches a polymer formed by reacting a high molecular weight polydimethyl siloxane diol with 2 mole equivalents isophorone diisocyanate and then reacting with excess hydroxy containing monomers. Essentially, this is a soft segment prepolymer endcapped with ethylenically reactive endcap. These materials are relatively weak and in their hydrated form show low degrees of elongation.
U.S. Pat. No. 4,309,526 teaches adhesive compositions which employ low molecular weight polyols reacted with diisocyanates and short chain cycloaliphatic or aromatic diols, endcapped with hydrophilic endcaps. Various characteristics such as oxygen permeability are not mentioned.
U.S. Pat. No. 4,359,553 teaches polyurethane diacrylate compositions useful as biomedical materials which are formed by reacting a diol mw 200 to 20,000 with 2 mole equivalents diisocyanate which is then reached with diethyleneglycol diacrylate to form the water soluble polyurethane. Specific applications taught in the patent are as controlled release matrices.
U.S. Pat. No. 4,454,309 teaches a hydrophilic random block copolymer with polyurethane linkage between randomly using high molecular weight polyols and low molecular weight ethylene mers. The materials absorbed 100-500% by weight additional water.
U.S. Pat. No. 4,740,533 teaches materials which are block copolymers of polyoxyalkylenes and polysiloxanes which have no hard segments.
U.S. Pat. No. 4,780,488 teaches a prepolymer with only a central soft segment endcapped with hydroxyethyl methacrylate endcaps or the like.
The art does not disclose polyurethane prepolymers useful as biomedical materials with the hard-soft-hard or the soft-hard-soft configurations disclosed herein which are oxygen permeable and still exhibit exemplary physical strength characteristics. Nor does the art teach that these prepolymers are useful as biomedical materials.